Sleep complaints are one of the most common health problems facing middle- and older-aged adults, resulting in significant medical utilization along with reduced psychological well-being and quality of life. In light of the cost and potential adverse effects accompanying long-term use of sedative-hypnotic medications, the development of alternative strategies for improving sleep in older populations is strongly indicated. One such potentially efficacious strategy is regular physical activity, a health behavior that typically diminishes with age. Although results from the first randomized, controlled trial evaluating the effects of increases in physical activity on sleep quality among healthy older adults with initial sleep complaints were promising, the study suffered from several methodological constraints that require further investigation. The objectives of this study are to: (1) determine the initial (6 months) and longer-term (12 months) effectiveness of a moderate-intensity physical activity program in promoting significant improvements in subjectively and objectively measured sleep quality in older adults with moderate sleep complaints; (2) evaluate the efficacy of the physical activity regimen for promoting initial and longer-term changes in a broad array of psychological and physical functioning (quality of life) variables; and (3) investigate a selected set of potential mediators and moderators of the physical activity/sleep relationship. The study design is a one-year randomized trial in which 120 sedentary women and men ages 55 years and older with moderate sleep complaints will be randomly assigned to one of two conditions: moderate-intensity physical activity training, or an attention-control (health education) condition. Both programs are designed to enhance long-term participation. Data will be collected at baseline and at 6 and 12 months using age-relevant physical performance and physical activity instruments, self-rated and objective sleep quality measures collected in the natural environment, and a comprehensive set of psychological and physical functioning scales. Our primary hypotheses are that subjects assigned to physical activity training will demonstrate significant increases in sleep quality along with other quality of life measures, maintained through 12 months, relative to the non-exercise control condition. This study will contribute important information with respect to the development of effective nonpharmacologic approaches to improving sleep quality among the large segment of healthy, community-dwelling older adults with sleep complaints.